Human-machine interfaces that are used to translate human movements to machine movements are used in myriad industries. For example, some aircraft flight control systems include a human-machine interface in the form of one or more control sticks, pedals, or other controls. The flight control system, in response to input forces supplied to the interface(s) from the pilot, controls the movements of various aircraft flight control surfaces. No matter the particular end-use system, the human-machine interface preferably includes some type of haptic feedback mechanism back through the interface to the interface operator.
Many haptic feedback mechanisms are implemented using a force sensor as the primary input device to the feedback loop. In most instances, the force sensor drives a servo amplifier, which in turn drives a motor. The motor, which is coupled to the human-machine interface via a gearbox, supplies a feedback force to the human-machine interface. Although these types of haptic feedback mechanisms are generally safe and reliable, they do suffer certain drawbacks. For example, the force sensor (or sensors) increases overall system cost and complexity. When redundancy is employed to increase overall system reliability, the system cost and complexity can be significant.
In addition to increased costs, the force sensor many times senses undesired high frequency vibratory force inputs from the human hand. These force inputs, when sensed, may be amplified, and tuning the feedback loop to reject these vibratory force inputs can adversely impact system characteristics. Moreover, the servo feedback loop can be difficult to tune for acceptable feel because of the high gain associated with a force sensor, and because the motor may be separated from the force sensor by the gearbox. As a result, in some designs additional sensors are used to sense motor velocity and/or angular acceleration, further adding to costs. The servo feedback loop typically needs relatively high mechanical stiffness to implement a relatively high performance servo loop, which further increase cost and weight. Additionally, the force sensor may detect accelerations and some systems require auxiliary accelerometers to counteract the unwanted inputs.